Comparative In Vitro Antifungal Activity of Essential Oils, Plant Extracts, and Commercial Biological Products Against Fusarium avenaceum and Alternaria alternata

Fungal pathogens, such as Fusarium spp. and Alternaria spp., cause substantial yield losses worldwide through root and fruit rot, wilting, and leaf and fruit spots, and increasing restrictions on chemical pesticides have intensified interest in sustainable alternatives such as essential oils, plant extracts, and microbial biocontrol agents. However, these three treatment categories are rarely compared directly under identical experimental conditions, which limits conclusions about their relative efficacy. This study directly compared, for the first time under the same in vitro conditions, the antifungal activity of peppermint and thyme essential oils, clove and rosemary plant extracts, and three commercial biological products (Mycostop, Asir Fruit, Aegis) against Fusarium avenaceum and Alternaria alternata using the agar incorporation method, with fungicidal versus fungistatic activity determined by reinoculation. Most treatments inhibited mycelial growth of both pathogens relative to the untreated control, although a few treatments (Asir Fruit at early DAI and lowest-concentration thyme EO) showed no inhibition or even slight stimulation of A. alternata growth. At 7 days after inoculation, thyme essential oil (1000 µL/L) and clove plant extract at all concentrations tested completely inhibited both pathogens, and reinoculation confirmed that clove extract alone was fungicidal, likely reflecting its high eugenol content and consequent irreversible membrane damage; all other treatments were fungistatic, consistent with reversible effects on membrane permeability. Peppermint essential oil (2000 µL/L) inhibited F. avenaceum and A. alternata by 96% and 73%, respectively, while rosemary extract was comparatively weaker (57% and 21%). Among the commercial products, Mycostop, which contains Streptomyces griseoviridis, was the most effective, plausibly reflecting sustained antagonistic activity beyond metabolite secretion alone. These findings indicate that clove extract and thyme oil match or exceed commercial biocontrol products in vitro and warrant evaluation under greenhouse and field conditions as candidates for integrated disease management.